The
high concrete floors of the library prevented ordinary bricks from being laid
in the corridors and classrooms, as originally planned, because they would have
significantly decreased the height of the interior of the building. It was decided
that thinner roof tiles would instead be used throughout the building's corridors
and in several smaller rooms. Dissatisfied with samples of tile provided by
Philadelphia paving companies in 1905, Lockwood de Forest mentioned the project
to Henry Chapman Mercer (1856-1930), who had recently begun to make a name for
his Doylestown-based Moravian Pottery and Tile Works. On August 5, 1905, De
Forest recommended that Thomas contact Mercer, who had shown an interest in
the commission.

The tile sample is not as good as the brick you are laying in the cloister.
I think you had better have Mr. Mercer come and see you. I send you his last
letter saying he wants to have a chance to estimate. He has common sense and
I am certain could work out something good and cheap with you.

Mercer's credentials included much more than common
sense. In the previous year, he had been awarded a grand prize for his handcrafted
tiles, many of which reproduced medieval designs, by the Department of Art at
the Louisiana Purchase Exposition in St. Louis. Mercer's scholarly knowledge
of historic European
architecture, interest in traditional tile production, and commitment to creating
pavements that meshed with the character of the building were close to Thomas's
own concerns.

Mercer persuaded Thomas and de Forest to adjust the original plainly linear
plan for the cloister pavements according to his own interlocking design of
rectangular common bricks and small square Moravian "plugs." For the
roof tiles that pave the interior hallways of the building, Mercer provided
a pattern that echoes the cloister walkways, and a simple, geometric washboard,
or baseboard, design. Thomas would later tell Mercer, "There is nothing
that we have in the library that has been admired more than the tile washboard."
Similar pavements were also used to pave the laboratories
of the building.

Most of the decorated tiles in the
upper and lower main vestibules of the library replicate designs found at medieval
monastic sites. In these two spaces at the central axis
of the building are some of the most characteristic Mercer design tiles, interspersed
with square reddish brown tiles. Among the designs are wyverns and foliated
circles based on tiles found at Castle Acre Priory in
Norfolk, England; a crossed lozenge enclosing a cinquefoil based on a tile found
at St. Cross in Hampshire, England; and a small castle modeled on a tile Mercer
had seen at the Hôtel de Cluny in Paris in 1900. These "plaques"
are often framed by a series of colored Maltese crosses and smallchequer
tiles, both also based on examples seen at the Hôtel de Cluny.

Hexagonal tiles with a randomly interspersed cinquefoil
motif pave the floors of the two small side vestibules of the library, located
at opposite ends of the building.These are based on pavements Mercer had seen
at Avignon, France, and predate the example that is found in the Hispanic Society
building's central court in New York City, a pavement he designed in 1909. Square
"French quarry" tiles provide the washboarding of these vestibules
and their steps. These are the same tiles that Mercer
had provided for his first important commission, the Isabella Stewart Gardiner
Museum's Dutch gallery, in 1901.

Speaking in 1905 on the points of difference and similarity among the various
pavements in the library, Mercer wrote, "I think . . . the general similarity
of the laying of the bricks showing a harmony in the whole pavement scheme will
be an advantage rather than otherwise." Thomas, however, also appreciated
the care that he took in creating a variety of designs for the campus, and in
a 1906 letter to him wrote, "We try to make each of our buildings as different
as possible, in order to keep from looking like an institution."